[Qemu-devel] [PATCHv2a] [RFC 8/7 (really)] Add prctl(PR_SET_TIMERSLACK, 1, ...) to reduce timer slack

[Alex Bligh <alex@alex.org.uk>]

Where supported, called prctl(PR_SET_TIMERSLACK, 1, ...) to
set one nanosecond timer slack to increase precision of timer
calls.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
---
[ Additional patch on the the end of the PATCHv2 series - I'll
resend if I have further comments with this reordered so it's
next to the ppoll introduction ]

 configure    |   18 ++++++++++++++++++
 qemu-timer.c |    7 +++++++
 2 files changed, 25 insertions(+)

diff --git a/configure b/configure
index b491c00..c8e39bc 100755
--- a/configure
+++ b/configure
@@ -2817,6 +2817,21 @@ if compile_prog "" "" ; then
   ppoll=yes
 fi
 
+# check for prctl(PR_SET_TIMERSLACK , ... ) support
+prctl_pr_set_timerslack=no
+cat > $TMPC << EOF
+#include <sys/prctl.h>
+
+int main(void)
+{
+    prctl(PR_SET_TIMERSLACK, 1, 0, 0, 0);
+    return 0;
+}
+EOF
+if compile_prog "" "" ; then
+  prctl_pr_set_timerslack=yes
+fi
+
 # check for epoll support
 epoll=no
 cat > $TMPC << EOF
@@ -3811,6 +3826,9 @@ fi
 if test "$ppoll" = "yes" ; then
   echo "CONFIG_PPOLL=y" >> $config_host_mak
 fi
+if test "$prctl_pr_set_timerslack" = "yes" ; then
+  echo "CONFIG_PRCTL_PR_SET_TIMERSLACK=y" >> $config_host_mak
+fi
 if test "$epoll" = "yes" ; then
   echo "CONFIG_EPOLL=y" >> $config_host_mak
 fi
diff --git a/qemu-timer.c b/qemu-timer.c
index 4cba055..53cb62c 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -37,6 +37,10 @@
 #include <poll.h>
 #endif
 
+#ifdef CONFIG_PRCTL_PR_SET_TIMERSLACK
+#include <sys/prctl.h>
+#endif
+
 /***********************************************************/
 /* timers */
 
@@ -367,6 +371,9 @@ void init_clocks(void)
         vm_clock = qemu_new_clock(QEMU_CLOCK_VIRTUAL);
         host_clock = qemu_new_clock(QEMU_CLOCK_HOST);
     }
+#ifdef CONFIG_PRCTL_PR_SET_TIMERSLACK
+    prctl(PR_SET_TIMERSLACK, 1, 0, 0, 0);
+#endif
 }
 
 uint64_t qemu_timer_expire_time_ns(QEMUTimer *ts)
-- 
1.7.9.5